2.5mm ring with 40amp cb quandry

[mcmainelectric]

Hello All
Recently did a pir and found a 2 ring circuits wired through a 40amp c/b one in 4.0mm and one in 2.5mm
I feel that the 4.0 mm is acceptable and tradition would say that 2.5 is wrong but thinking about it if a 2.5 has a cuurent capacity of 24amps then on a ring circuit with half the current flowing down each leg then it is technically ok (before ambients grouping etc) to have a 40 amp although it is sailing close to the wind and taking into account a fully balanced ring which you cant, any discussion appreciated, i suppose the same logic would say a 32amp on a 1.5 ring is okay ( iknow it is not but cant get my head round the physics) if a balanced circuit was sending 16 amp down each side. Not really intrested in a osg recommendation more the physics
cheers

 

[Octopus]

Are you saying there are 4 cables into the MCB or 2 rings in seperate MCB's

Either way if you're doing a PIR IMHO its a code 2.

 

[yellowvanman]

433.1.5 will answer your question(s)

Current will not be split 50:50 around the legs. Imagine the 12 points of a clock as the ring, with a socket at each point. At 1 o'clock if pulling 13A most of that will go direct to 12 o'clock whilst a small proportion will go around via 6 o'clock. Similarly for 2 o'clock and 3 o'clock etc.

The above assume CU @ 12 o'clock

 

[johnboy6083]

isnt the ruile of thumb with ring finals 1.67. so for example i have a 2.5 conductor that is capable of carrying 20A over a reasonable distance. multiply that by 1.67 and you get 32A

 

[Chr!s]

I would read Reg 433.1.5

Regards Chris